2. Field of the Invention
This invention relates generally to apparatus for fusing toner images to copy sheets and more particularly to such apparatus including means for controlling the surface temperature of a heated pressure roller.
2. Description of the Prior Art
Electrographically reproduced copies generally comprise toner images of an original document, the toner images being permanently affixed to copy sheets by well known fusing techniques. One such technique consists of passing a copy sheet having toner images on one or both sides thereof through the nip of a pair of fuser rollers, one or both of which are heated to permanently fuse the toner image or images to the copy sheet through the application of heat and pressure. Generally the source for heating one or both of the fuser rollers comprise quartz lamps inserted inside the fuser rollers or a heating element which applies heat to the external surface of the rollers.
The fusing temperature should be maintained within preferred temperature limits dependent, for example, upon the type of toner to be fused, the characteristics of the fuser rollers, and the characteristics of the copy sheet material. Sensors are used to monitor the heat level of the fuser rollers in order to maintain the temperature within predetermined limits and to signal overheating so that the fuser roller may be shut down.
Skive means are supported in contact with the fuser roller at or near to the roller nip exit to assist in stripping copy sheets from the fuser rollers.
There are several prior art systems for providing heat transfer from the roller to the temperature sensor. In U.S. Pat. No. 4,043,747, which issued Aug. 23, 1977 to M. Ogiwara, a temperature detector for controlling a heating element rides against the surface of a heated fusing roller to directly measure the temperature of the roller surface. However, over a long time period, the surface of the heated roller is subject to abrasion from the detector, and the detector is subject to wear from the roller. If abraded, the roller is less likely to produce homogeneous fixing of the toner particles. To more evenly distribute the wear on the roller, the patent teaches moving the detector axially along the surface of the roller.
Non-contact temperature sensors overcome the abrasion problem of the previously mentioned prior art systems. Such non-contact sensors may be infrared temperature detectors which require complex electronics and increase the cost of the system. Another non-contact sensor is a thermocouple-type mounted within the thermal boundary layer adjacent the heated roller for sensing the temperature change of the air within the boundary layer and for generating an electrical signal varying in accordance with the temperature changes. However, such electrical signal is highly sensitive to changes in the spacing of the detector from the roller and to ambient temperature changes.
Another type of temperature sensing means known in the prior art is shown in an article (No. 19642) entitled "Temperature Control of Fuser Roller", published in Research Disclosure, August, 1980, pages 338 and 339. A thermistor is embedded in a graphite block which rides in contact with the heat conductive core of a fuser roller. The temperature sensed by the assembly produces an electrical signal to control the heat of the roller. Such systems which measure the temperature of the core are considered to be less accurate and to have longer time constants (unacceptable time lag in temperature controlling characteristics) than systems which directly measure the surface temperature of the roller.
In reviewing the above, it is recognized that temperature sensors which directly contact the surface of the roller are regarded as being the most accurate and least expensive of the known systems, but with the disadvantage of subjecting the roller surface to adverse abrasion. It is therefore a primary object of the invention to provide a temperature sensing system with the advantages of a contact sensor but which will not increase roller wear.